1. Field of the Invention
The invention relates to storage tanks generally, and more particularly to a high strength rib that can be used in a double-walled storage tank.
2. Discussion of the Background
Underground storage tanks are commonly used for the storage of liquids, particularly gasoline and other petroleum products. These tanks are generally cylindrical in shape, with either curved (e.g. hemispherical) or flat ends. Underground storage tanks may be made of many materials, including steel and fiber reinforced plastic (referred to herein as FRP and fiberglass). These tanks may be single, double, or multi-walled. Double or multi-walled tanks are required by many municipalities in situations in which gasoline or other environmentally harmful materials are stored in the tanks. As used herein, a xe2x80x98double walled tankxe2x80x99 is a term of art that refers to a tank that includes an inner wall, an outer wall and an annular space between the inner and outer walls. The annular space in such tanks is generally used to monitor the inner and outer walls of the tank for cracks and other damage. Known monitoring systems include wet, dry, pressure and vacuum systems. All of these systems are well known in the art and will not be discussed in further detail herein.
Because these tanks are underground, they are subjected primarily to compressive forces exerted on the tank by the surrounding fill (sand, pea gravel, or the like). These forces can be even greater when an underground storage tank is installed at a location such as a service station where vehicles may drive over the tank.
In order to provide underground storage tanks with sufficient structural strength to withstand these compressive forces, ribs are often provided with the tank. These ribs may be integral; that is, they are formed in the tank wall. Forming integral ribs presents some challenges. First, forming internal integral ribs is difficult using male molding technology since a collapsible mandrel is required. Second, when integral ribs are made from the same material as the tank walls, the ribs are limited in their specific strength.
What is needed is a high strength rib and an underground storage tank that incorporates such a rib that can be easily and inexpensively manufactured.
The present invention meets the aforementioned need to a great extent by providing a high strength rib featuring a xe2x80x9cYxe2x80x9d cross sectional shape and high modulus reinforcing material such as steel or graphite in the body of the rib. The present invention also provides a method for manufacturing a double walled underground storage tank using the high strength rib as well as a method for manufacturing the rib itself.
In one aspect of the invention, the xe2x80x9cYxe2x80x9d cross sectional shape of the rib provides a channel between the branches of the xe2x80x9cYxe2x80x9d. This channel may be used as part of an annular monitoring space in a double walled or multiple walled tank.
In another aspect of the invention, the rib may be manufactured separately from the tank and installed on the interior of the tank after the tank has been completed. This allows the construction of an internally ribbed tank using a male mold without the necessity of providing a collapsible mandrel.
In another aspect of the invention, the rib may be manufactured separately from the tank and installed on the exterior of the tank after the tank has been completed. Alternatively, the rib may be formed integrally with an outer wall after the tank,
In still another aspect of the invention, a method of manufacturing such a high strength rib employs a rotating cylindrical male mold having a helical channel with a xe2x80x9cYxe2x80x9d cross sectional shape, the trunk of the xe2x80x9cYxe2x80x9d being at the bottom of the channel and the branches of the xe2x80x9cYxe2x80x9d being at the top of the channel (the outermost surface of the cylindrical male mold). While the cylindrical mold rotates, material such as fiberglass is deposited into the channel. This material may include high modulus materials such as graphite elements and/or metal reinforcing rods, which are preferably placed in the trunk section of the xe2x80x9cYxe2x80x9d shaped channel. When the channel has been filled and any necessary drying/curing has completed, the material in the channel is cut in a direction transverse to the channel along the length of the cylindrical mold such that individual sections of the material can be removed from the mold and be made into hoops (with an outside diameter equal to the inside diameter of the tank) by attaching the cut ends of a section to each other. Preferably, to facilitate installation into a tank, the cut ends are not attached until the rib has been positioned at a desired location inside the tank. Additional materials may then be deposited over the rib and interior tank wall to secure the rib to the tank wall.
In one preferred embodiment of a fiberglass underground storage tank incorporating the high strength rib, an outer wall is constructed using a female mold. After the outer wall has cured, a plastic film such as Mylar(copyright) is placed over the inside surface of the outer wall, including areas where ribs are to be installed. Plastic films such as Mylar(copyright) will allow fluids to flow between two tank walls that are separated by such a film, as discussed in commonly owned U.S. Pat. No. 5,720,404, the contents of which are incorporated by reference herein. In alternative embodiments, a three dimensional distance fabric, which is preferably loadtransmitting, that allows the passage of fluids therethrough, may be used in place of or in combination with the film. (In still other embodiments, the surface of the walls may be manufactured with protrusions that creates spaces between the walls through which fluid may pass even when walls are adjacent to each other.) Ribs are then placed over the film or distance fabric. Next, an inner wall is layed up over the film or material and ribs. If a third or more walls are required, an additional layer(s) of film/fabric and an additional wall may be formed over the aforementioned structure.
In another preferred embodiment of the invention, the branches of a xe2x80x9cYxe2x80x9d shaped rib have a top portion with a width less than a specified maximum thickness. This allows the rib to be attached directly to the inside surface of the outer wall. A plastic film or distance fabric is then placed over the portions of the inside surface not covered by the ribs such that the film/fabric abuts the outside surfaces of the branches of the xe2x80x9cYxe2x80x9d shaped rib. Along one or more discrete regions along the circumference of the tank, a gutter (a channel formed between the inner and outer walls) is formed along the length of the tank and is in fluid communication with both the annular spaces formed on the inside of the branches of the xe2x80x9cYxe2x80x9d shaped rib and the outer wall and the annular spaces formed by the film/fabric between ribs. This arrangement ensures that a single annular space is present everywhere between the inner and outer walls, save for those points at which the branches of the xe2x80x9cYxe2x80x9d shaped rib are attached to the outer wall. As the thickness of the branches of the xe2x80x9cYxe2x80x9d shaped rib are less than the maximum allowable distance at all of these points, the aforementioned double wall tank requirement is met. In situations where no point through which a line may pass at an angle normal to the outer surface of the tank without passing through an annular space is allowed, the film or material may be extended up over the branches of the xe2x80x9cYxe2x80x9d shaped rib such that the ends of the film/material extend past the point at which the branches are attached to the tank wall.